Thermoelectric conversion material and thermoelectric conversion device

ABSTRACT

The invention provides a thermoelectric conversion material which is low toxic and can be used at a high temperature of 500° C. or higher without variation in performance, and a thermoelectric conversion device containing the material. The thermoelectric conversion material is formed of an oxide represented by (Ca 3-x M x )Co 4 O 9  (M: Sr or Ba, 1.2&gt;x&gt;0.5).

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a thermoelectric conversionmaterial to be employed at high temperature for so-called thermoelectricconversion (i.e., direct energy conversion without use of any movableparts), including power generation on the basis of Seebeck effect andelectronic freezing on the basis of Peltier effect. The invention alsorelates to a thermoelectric conversion device containing the material.

[0003] 2. Background Art

[0004] Thermoelectric conversion by use of a thermoelectric conversionmaterial; e.g., thermoelectric power generation or electronic freezing,finds utility in a simplified direct-energy-conversion apparatus havingno movable parts that generate vibration, noise, wear, etc.; having asimple, reliable structure; having a long service life; and facilitatingmaintenance. Thus, thermoelectric conversion is suitable for directgeneration of DC power without combustion of a variety of fossil fuelsor other sources and for temperature control without use of a coolingmedium.

[0005] Characteristics of thermoelectric conversion materials areevaluated on the bases of power factor (Q) and figure of merit (Z),which are represented by the following formulas:

Q=σα²  [Formula 1]

[0006] $\begin{matrix}{Z = \frac{\sigma \quad \alpha^{2}}{\kappa}} & \text{[Formula~~2]}\end{matrix}$

[0007] wherein α represents Seebeck coefficient; σ represents electricconductivity; and κ represents thermal conductivity. Thermoelectricconversion materials are desired to have a high figure of merit (Z);i.e., a high Seebeck coefficient (α), high electric conductivity (σ),and low thermal conductivity (κ).

[0008] For example, when employed for thermoelectric conversion such asthermoelectric power generation, a thermoelectric conversion material isdesired to have a figure of merit as high as Z=3×10⁻³ 1/K or higher andto operate without variation for a long period of time under varyingoperation conditions. Mass production of thermoelectric power generatorsfor use in vehicles or employing discharged heat gives rise to demandfor a thermoelectric conversion material which has sufficiently highheat resistance and strength, particularly at high temperature, andresistance to deterioration in characteristics, as well as a method forproducing the material at high efficiency and low cost.

[0009] Conventionally, PbTe or silicide materials including silicidecompounds such as MSi₂ (M: Cr, Mn, Fe, or Co) and mixtures thereof havebeen used to serve as the aforementioned thermoelectric conversionmaterials.

[0010] Sb compounds such as TSb₃ (T: Co, Ir, or Ru) have also been used.For example, there has been disclosed a thermoelectric material whichcomprises a material containing CoSb₃ as a predominant component and animpurity added for determination of conduction type (L. D. Dudkin and N.Kh. Abriko Sov, Soviet Physics Solid State Physics (1959) p. 126; B. N.Zobrinaand, L. D. Dudkin, Soviet Physics Solid State Physics (1960) p.1668; and K. Matsubara, T. Iyanaga, T. Tsubouchi, K. Kishimoto, and T.Koyanagi, American Institute of Physics (1995) p. 226-229).

[0011] Thermoelectric conversion materials formed of PbTe exhibit a highfigure of merit (Z)—an index of thermoelectric properties—ofapproximately 1×10⁻³ 1/K at about 400° C. However, the materials have alow melting point and poor chemical stability attributed to Te, avolatile component contained in the materials, and cannot be used athigh temperature of 500° C. or higher. In addition, since cumbersomeproduction steps are required due to presence of a volatile Te componentin the materials, variation in product characteristics tends to becaused by variation in composition of the materials, failing to attaineffective mass-production. Another disadvantage is that the rawmaterials for producing the thermoelectric conversion materials areexpensive and highly toxic.

[0012] Silicide materials including silicide compounds such as MSi₂ (M:Cr, Mn, Fe, or Co) and mixtures thereof can be produced from inexpensiveraw materials; contain no toxic components; are chemically stable; andcan be used at temperatures of about 800° C. “Netsuden Handotai To SonoOyo,” authored by Kunio NISHIDA and Kin-ichi UEMURA, (1983) p. 176-180(published by Nikkan Kogyo Shimbun) discloses a comparativelyinexpensive method of producing these silicide materials. However, thesesilicide materials exhibit a thermoelectric property (e.g., a figure ofmerit (Z) of approximately 1-2×10⁻⁴ 1/K) of about one-tenth that of PbTeand cannot provide sufficient thermoelectric properties comparable tothose of PbTe.

[0013] Thermoelectric materials containing an Sb compound such as TSb₃(T: Co, Ir, or Ru) as a predominant component (e.g., CoSb₃) are producedfrom non-toxic, comparatively inexpensive raw materials and are known toexhibit a comparatively high figure of merit (<1×10⁻³ 1/K).

[0014] In the production of a conventionally known thermoelectricconversion material having a chemical composition of CoSb₃, it isdesirable that cubic CoSb₃ crystal phase is exclusively formed in thematerial to serve as a constitutional crystal phase, with other crystalphases (CoSb, CoSb₂, and Sb), which are detrimental to thermoelectricproperties, being removed. However, in reality, when a production methodinvolving melting CoSb₃ is employed, undesired phases (CoSb, CoSb₂, andSb) other than CoSb₃ phase are precipitated during the course ofsolidification. In order to generate a crystal phase formed only ofCoSb₃ from such a molten material, heat treatment at approximately 600°C. for about 200 hours is required, and such treatment disadvantageouslyprolongs the time required for production steps.

[0015] In addition, when a production method in which a solidified CoSb₃melt is pulverized and sintered is applied, undesired phases (CoSb,CoSb₂) precipitated during solidification and having a higher densitythan that of CoSb₃ are transformed into CoSb₃ phase during firing. Thisphase transformation causes volume expansion, thereby disadvantageouslyinhibiting sintering. Specifically, sufficiently densified material hasnever been produced, even when pulverized CoSb₃ is hot-pressed at 5×10³kg/cm² and 600° C. (Reference: K. Matsubara, T. Iyanaga, T. Tsubouchi,K. Kishimoto, and T. Koyanagi, American Institute of Physics (1995) p.226-229). The maximum density of the thus-sintered CoSb₃, reported inthe reference, is 5.25 g/cm³, whereas the theoretical density of cubicCoSb₃ is 7.64 g/cm³. Thus, the sintered CoSb₃ is a considerably fragilematerial, and has poor strength at high temperature.

[0016] In order to attain satisfactory durability of a material formedof heavy elements such as Bi, Te, Se, and Pb against contact withindustrial process discharge gas and to prevent vaporization ofconstitutional components in a high-temperature reaction atmosphere andcontamination with the vaporized components, there has been desired anew material which can be produced at low cost; causes lessenvironmental pollution; and can be used without causing variation evenat high temperature.

[0017] In view of the foregoing, a strong tendency to use an oxide as athermoelectric material has rapidly arisen. Generally, an oxide has lowmobility and a typical carrier concentration of about 10¹⁹ cm⁻³,exhibiting no conductivity, unlike a metal. Thus, it has been commonlyaccepted in the art that an oxide cannot serve as thermoelectricconversion material. However, in 1997, an oxide of layer structure,NaCo₂O₄, was surprisingly found to exhibit strong thermoelectromotiveforce despite its low resistivity (Japanese Patent Application Laid-Open(kokal) No. 2000-211971). Thermoelectric properties of this class ofoxide are remarkably superior to those of other oxides, and approachthose of existing thermoelectric material used in practice.

[0018] However, this oxide also has a drawback in that thermoelectricproperties of products vary greatly in accordance with productionconditions, due to sublimation of Na during sintering. In addition, whenthis oxide is used at high temperature, sublimation of Nadisadvantageously deteriorates thermoelectric properties, and when theoxide is allowed to stand in air, resistivity problematically increases.Furthermore, Na is highly reactive to water contained in air, and theresultant reaction may deteriorate performance of products.

SUMMARY OF THE INVENTION

[0019] The present inventors have carried out extensive studies in orderto solve the aforementioned problems, and have found a novel oxidethermoelectric conversion material. The present invention has beenaccomplished on the basis of this finding.

[0020] Thus, an object of the present invention is to provide athermoelectric conversion material which has low toxicity and can beused at high temperature of 500° C. or higher without variation inperformance. Another object of the present invention is to provide athermoelectric conversion device containing the material.

[0021] Accordingly, in one aspect of the present invention, there isprovided a thermoelectric conversion material formed of an oxiderepresented by (Ca_(3-x)M_(x))Co₄O₉ (M: Sr or Ba, 1.2>x>0.5).

[0022] The oxide may be oriented along the C axis.

[0023] In another aspect of the invention, there is provided athermoelectric conversion device containing the thermoelectricconversion material.

[0024] In still another aspect of the invention, there is provided amethod of thermoelectric conversion comprising effecting thermoelectricconversion by use of a thermoelectric conversion material.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Various other objects, features, and many of the attendantadvantages of the present invention will be readily appreciated as thesame becomes better understood with reference to the following detaileddescription of the preferred embodiments when considered in connectionwith accompanying drawings, in which:

[0026]FIG. 1 is a schematic view of the crystal structure of a typicalthermoelectric conversion material according to the present invention;

[0027]FIG. 2 is a chart showing X-ray diffraction patterns ofthermoelectric conversion materials produced in accordance with Examplesand Comparative Examples of the present invention;

[0028]FIG. 3 is a chart showing X-ray diffraction patterns ofthermoelectric conversion materials produced in accordance withComparative Examples of the present invention;

[0029]FIG. 4 is a graph showing temperature dependence of Seebeckcharacteristics, at low temperatures, of thermoelectric conversionmaterials produced in accordance with Examples and Comparative Examplesof the present invention;

[0030]FIG. 5 is a graph showing temperature dependence of Seebeckcharacteristics, at high temperatures, of thermoelectric conversionmaterials produced in accordance with Examples and Comparative Examplesof the present invention;

[0031]FIG. 6 is a graph showing temperature dependence of resistivity,at low temperatures, of thermoelectric conversion materials produced inaccordance with Examples and Comparative Examples of the presentinvention;

[0032]FIG. 7 is a graph showing temperature dependence of resistivity,at high temperatures, of thermoelectric conversion materials produced inaccordance with Examples and Comparative Examples of the presentinvention;

[0033]FIG. 8 is a graph showing temperature dependence of output factor,at low temperatures, of thermoelectric conversion materials produced inaccordance with Examples and Comparative Examples of the presentinvention; and

[0034]FIG. 9 is a graph showing temperature dependence of output factor,at high temperatures, of thermoelectric conversion materials produced inaccordance with Examples and Comparative Examples of the presentinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0035] The thermoelectric conversion material of the present inventionformed of an oxide represented by Ca₃Co₄O₉ in which a portion of Caatoms are substituted by predetermined amounts of Sr atoms or Ba atoms.Thus, advantageously, the material is low-toxic and can be used at atemperature as high as 900° C. without substantial variation inperformance.

[0036]FIG. 1 shows the structure of the (Ca_(3-x)M_(x))Co₄O₉ accordingto the present invention. As shown in FIG. 1, the oxide has a structurein which CoO₂ layers 11 and Ca₂CoO_(4-δ) layers 12 are alternatelystacked. Each CoO₂ layer 11 consists of a plurality of CoO₆ octahedronsin which each boundary line is shared by adjacent octahedrons. EachCa₂CoO_(4-δ) layer 12 consists of a plurality of CoO₆ octahedrons inwhich each apex is shared by two adjacent octahedrons. In theCa₂CoO_(4-δ) layer 12, a portion of Ca atoms are substituted by Sr atomsor Ba atoms.

[0037] The thermoelectric conversion material of the present inventionis formed of (Ca_(3-x)M_(x))Co₄O₉, and the oxide has a layer structurein which CoO₂ layers 11 and Ca₂CoO_(4-δ) layers 12 are stacked. Eachunit lattice has a plurality of crystallographically sites which areindependent from one another, and presence of these sites enablesprovision of high electric conduction and low thermal conduction,attaining excellent thermoelectric properties. A more specific mechanismis conceived such that the sites forming the skeleton of the crystalstructure form a band structure which determines electric conductivityand the other sites induce scattering of phonons relating to heattransportation, to thereby greatly reduce the effects of lattices onthermal conduction.

[0038] Although NaCo₂O₄, disclosed in Japanese Patent ApplicationLaid-Open (kokai) No. 2000-211971, also has CoO₂ layers, the layerstructure thereof comprises alternately stacked CoO₂ layers andNa-50%-deficient layers. Thus, this structure differs from that of the(Ca_(3-x)M_(x))Co₄O₉ according to the present invention.

[0039] The (Ca_(3-x)M_(x))Co₄O₉ which forms the thermoelectricconversion material of the present invention is preferably orientedalong the C-axis. This is because thermoelectric properties are improvedon the basis of anisotropy in physical properties, particularly electricresistivity, between the lateral direction and the perpendiculardirection, the anisotropy being attributed to the aforementioned layerstructure. Any known method such as hot-pressing or plasma discharge canbe applied to effect orientation in the C-axis direction.

[0040] The (Ca_(3-x)M_(x))Co₄O₉ which forms the thermoelectricconversion material of the present invention is also expected to providelarge thermoelectromotive force and low resistivity. In other words,electron states similar to those of the heavy electron system areconceived to be realized in Ca₃Co₄O₉—an oxide of layered structure. Inthe oxide, a Hubbard band sustaining localized spin and a wideneditinerant-electron band are present. When an interaction (magnitude ofk_(B)T_(K)), called the Kondo effect, is applied between these twobands, coherent states having an energy gap of k_(B)T_(K), which iselectron states involving the interaction between these two bands, areformed. In such states, electrons move accompanying a spin wave and at aspeed approximately as slow as that of the spin wave. Such electronstates correspond to an increase in effective mass of an electron, andthermal electromotive force represented by E_(F)/k_(B)T_(K) increases asshown in the below-shown formula (1). In addition, electrons are notscattered by the spin wave, and scattering time is prolonged, to therebysuppress resistivity. Thus, in the coherent state, particularly largethermal electromotive force and low resistivity are expected.

[0041] Among strong correlation electron system oxides, conduction inthe (Ca_(3-x)M_(x))Co₄O₉ which forms the thermoelectric conversionmaterial of the present invention is conceived to be a type of hoppingconduction of small polarons, and conductivity σ and thermalelectromotive force α are represented by the following formulas (2) to(4): $\begin{matrix}\text{[formula~~3]} \\\begin{matrix}{{Z = \frac{4Z_{p}Z_{n}}{\left( {\sqrt{Z_{p}} + \sqrt{Z_{n}}} \right)^{2}}}\text{[formula 4]}} & (1) \\{{\sigma = {\left( \frac{C}{T} \right){\exp \left( {- \frac{E}{k\quad T}} \right)}}}\text{[formula~~5]}} & (2) \\{{C = \frac{{r\left( {1 - r} \right)}e^{2}a^{2}{Nv}}{k}}\text{[formula~~6]}} & (3) \\{\alpha = {\left( \frac{k}{e} \right){Ln}\left\{ \frac{2\left( {1 - r} \right)}{r} \right\}}} & (4)\end{matrix}\end{matrix}$

[0042] wherein E represents activation energy for causing hoppingconduction; k represents Boltzmann constant; r represents the ratio ofpolaron density to density of sites available for hopping; e representselementary electric charge; a represents inter-hopping-site spacing; Nrepresents density of sites available for hopping; and v representsfrequency of lattice vibration (optical mode). When hopping occurs amongsites unequivalent to one another, a term proportional to temperature isappended to the right side of formula (4). Substances which allowhopping conduction; e.g., Ca₃Co₄O₉ are expected to attain greaterthermal electromotive force and lower resistivity by reducing activationenergy for causing hopping conduction or by increasinginter-hopping-site spacing through element substitution or othertechniques.

[0043] The (Ca_(3-x)M_(x))Co₄O₉ which forms the thermoelectricconversion material of the present invention exhibits excellentthermoelectric properties on the basis of substitution of predeterminedamounts of calcium atoms occupying Ca sites in Ca₃Co₄O₉ by strontiumatoms or barium atoms. This is conceived to be attributable to anincrease in carrier mobility caused by substitution of calcium ions bystrontium ions or barium ions having an ionic radius greater than thatof calcium ion.

[0044] The degree of substitution of calcium ions, represented by x in(Ca_(3-x)M_(x))Co₄O₉, preferably falls within the range of: 1.2>x>0.5.This is because when x is 1.2 or greater, a crystal phase formed only ofa specific component cannot be formed and resistivity tends to increase,whereas when x is 0.5 or less, thermoelectric properties deteriorate.

[0045] No particular limitation is imposed on the element of atoms bywhich calcium ions are substituted, and any element can be used so longas the element enhances thermoelectric properties upon substitution ofcalcium ions. Other than Sr and Ba, elements such as Bi, La, and Y maybe used.

[0046] By use of the thermoelectric conversion material of the presentinvention as described hereinabove, a thermoelectric conversion deviceproviding excellent thermoelectric properties can be fabricated. Theconstitution of the device is not particularly limited, and any ofconventionally known structures may be employed. For example, the devicemay be of a type in which electromotive force is obtained from thedifference in temperature or a heat-pump for cooling or heating throughapplication of electric power.

EXAMPLES

[0047] The present invention will next be described in detail by way ofexamples, which should not be construed as limiting the inventionthereto.

Examples 1 and 2

[0048] CaCO₃ powder(purity: 99.99%) , Co₃O₄ powder(purity: 99.9%), andSrCO₃ powder (purity: 99.9%) were weighed at proportions (stoichiometriccomposition) so as to attain x in (Ca_(3-x)M_(x))Co₄O₉ of 0.75(Example 1) or 1.0 (Example 2). The raw materials and ZrO₂ balls(diameter: 5 mm) were placed in a polypropylene-made container, andwet-mixed for 24 hours by use of ethanol as a dispersion medium. Theresultant powder was dried and granulated by use of a sieve (mesh: 106μm). Each granulated product sample (approximately 3 g) was compressedby use of a rectangular-press machine (preliminary compression at 250kgf/cm² for 2 minutes and subsequent compression at 1,000 kgf/cm² for 5minutes). The resultant compact was heated under oxygen at 5° C./min to900° C. and fired at 900° C. for 10 hours, to thereby produce a sinteredrod having a square cross-section (about 45 mm×3.4 mm×5 mm).

Examples 3 and 4

[0049] Each of the sintered products produced in Examples 1 and 2 washot-pressed by means of a vacuum hot-press machine (product of TokyoShin-ku) and medium powder for two hours at 800° C. and 100,150 kg/cm²,to thereby produce a sintered product.

Comparative Examples 1 to 3

[0050] The procedure of Example 1 was repeated, except that x in(Ca_(3-x)M_(x)) Co₄O₉ was changed to 0 (Comparative Example 1), 0.5(Comparative Example 2), or 1.2 (Comparative Example 3), to therebyproduce each sintered product.

Test Example 1 XRD Measurement

[0051] Each of the sintered products of Examples 1 to 4 and ComparativeExamples 1 to 3 was identified through X-ray diffraction analysis bymeans of an analyzer (MXP³, product of Mac Science; Cu target).Measurement was performed under the following conditions:

[0052] Measurement range: 5.0-60.6 deg;

[0053] Sampling intervals: 0.02 deg;

[0054] Scanning speed: 3.0/min;

[0055] Measurement method: Typical method (without BG measurement);

[0056] Voltage generated: 40 kV;

[0057] Current generated: 30 mA;

[0058] Divergent slit: 1.0 deg;

[0059] Scatter slit: 1.0 deg; and

[0060] Emission slit: 0.15 mm.

[0061]FIGS. 2 and 3 show the results.

[0062] The results indicate that calcium cobalt oxides in which Ca atomsare partially substituted by Sr atoms and which have been produced inExamples 1 and 2 assume a crystal phase formed only of the correspondingoxide component. The results also have revealed that diffraction peakswere shifted to smaller in angle as the amount of Sr atoms increased. Inthe sample produced in Comparative Example 3 in which calcium atoms wereexcessively substituted by Sr atoms (x=1.2), no formation of a crystalphase formed only of a specific component was identified.

Test Example 2

[0063] Each of the sintered products of Examples 1 and 2 and ComparativeExamples 1 and 2 was analyzed in terms of dependence of Seebeckcoefficient on temperature. Measurement was performed in the followingmanner.

[0064] In the case of T<300 K

[0065] A test piece cut from each sample was fixed on two copper platesby use of a silver paste such that the test piece was brought intocontact with both copper plates. While the test piece was graduallycooled from room temperature by use of liquid helium, temperaturedifference (ΔT, approximately 5 K) was induced between two fixationpoints of the test piece, by heating one copper plate by means of aheater connected to the copper plate, and the electromotive force wasmeasured. Upon measurement, Cu, Cu-Ct, Cernox thermometer, and a straingauge were employed as measuring wire, a thermocouple, a thermometer,and a heater, respectively.

[0066] In the case of T>300 K

[0067] The procedure of the above case (T<300 K) was repeated, exceptthat temperature difference (ΔT, approximately 20 K) was induced betweenboth ends of the test piece through employment of temperature gradientin a tube furnace that was provided by placing a heater at one end ofthe furnace.

[0068] As shown in FIGS. 4 and 5, the samples of Examples 1 and 2exhibited, both at low temperatures and high temperatures, a Seebeckcoefficient higher than that of the sample of Comparative Example 1formed of conventional Ca₃Co₄O₉. The Seebeck coefficient of the sampleof Comparative Example 2 (x=0.5) was found to be lower than that ofCa₃Co₄O₉.

Test Example 3

[0069] Dependence of resistivity on temperature of each of the sinteredproducts of Examples 1 and 2 and Comparative Examples 1 and 2 wasmeasured through the DC four-probe method.

[0070] As shown in FIGS. 6 and 7, the samples of Examples 1 and 2exhibited, both at low temperatures and high temperatures, a resistivitylower than that of the sample of Comparative Example 1 formed ofconventional Ca₃Co₄O₉. Particularly, the resistivity at hightemperatures of the sample of Example 2 (x=1.0) was found to beapproximately ½ that of the sample of Comparative Example 1.

Test Example 4

[0071] Dependence of output factor on temperature of each of thesintered products of Examples 1 and 2 and Comparative Examples 1 and 2was measured. FIGS. 8 and 9 shows the results. The output factor wascalculated by the formula: P=S²/ρ. (S represents Seebeck coefficient.)

[0072] The samples of Examples 1 and 2 exhibited, both at lowtemperatures and high temperatures, an output factor higher than that ofthe sample of Comparative Example 1 formed of conventional Ca₃Co₄O₉. TheSeebeck coefficient and resistivity of the sample of Example 2 (x=1.0)were remarkably enhanced, and the maximum output factor was found to be3.8 μW/K²cm at 700K.

[0073] As described hereinabove, the present invention provides athermoelectric conversion material which is low toxic and can be used ata high temperature of 500° C. or higher without variation inperformance, and a thermoelectric conversion device containing thematerial.

What is claimed is:
 1. A thermoelectric conversion material formed of anoxide represented by (Ca_(3-x)M_(x))Co₄O₉ (M: Sr or Ba, 1.2>x>0.5).
 2. Athermoelectric conversion material according to claim 1, wherein theoxide is oriented along the C axis.
 3. A thermoelectric conversiondevice containing a thermoelectric conversion material as recited inclaim
 1. 4. A thermoelectric conversion device containing athermoelectric conversion material as recited in claim
 2. 5. A method ofthermoelectric conversion comprising effecting thermoelectric conversionby use of a thermoelectric conversion material as recited in claim
 1. 6.A method of thermoelectric conversion comprising effectingthermoelectric conversion by use of a thermoelectric conversion materialas recited in claim 2.